1. Field of the Invention
The present invention relates to a variable frequency power unit system for driving a rotary machine such as a fan or a pump by a variable frequency power unit (hereinafter referred to as "VF PWU") and more particularly to a VF PWU operating system for controlling a VF PWU in case an abnormal control signal is given to the VF PWU due to an accident such as disconnection or short-circuit.
2. Description of the Prior Art
A conventional VF PWU system of this kind is shown in FIG. 1, in which there are shown a commercial power supply line 1, a VF PWU 2B, a motor 3, a rotary machine 4, a control unit 5A, a control signal 6, a control signal line 7A for sending a control signal from the control unit 5A to the VF PWU 2B, a coupling device 8 for mechanically coupling the motor 3 and the rotary machine 4, and a switch 9.
In FIGS. 2 and 3, t.sub.1 and t.sub.3 depict times when an increase command and a decrease command are given by means of the control signal 6 respectively, t.sub.2 and t.sub.4 times when the output of the VF PWU 2B has completed the increase and the decrease, respectively, t.sub.5 time when an accident occurred in the signal line 7A and t.sub.6 time when the change of the output of the VF PWU 2B is completed.
The manner of operation of the VF PWU system will be described hereinbelow. In order to describe the manner of operation concretely, the rotary machine 4 is supposed to be a fan which provides an output (air flow) supplied to a boiler, not shown, installed in a power plant.
Referring to FIG. 1, electric power is supplied from the commercial power supply line 1 through the switch 9 to the VF PWU 2B and the VF PWU 2B provides an output to drive the motor 3. The revolving rate n of the motor 3 is defined by ##EQU1## where n is revolving rate of the motor, F is the frequency of the power supplied to the motor and P is the number of poles of the motor.
As clear from equation (1), the revolving rate n is proportional to the frequency F of the power supplied to the motor, namely, the output frequency F of the VF PWU 2B, and hence the revolving rate n of the motor 3 varies according to the output frequency F.
The motor 3 and the fan 4 are coupled directly to each other through a coupling 8 and the fan supplies air to the boiler at a rate approximately proportional to the revolving rate n of the motor 3.
In the power plant, when the power demand of the associated power grid changes or the fuel supply rate of the boiler changes, the air demand Q of the boiler, i.e., air supply rate Q, changes accordingly. In such a case, a control command 6 to change the air supply rate Q is given to the control unit 5A, and then the control unit 5A gives a command signal through the signal line 7A to the VF PWU 2B. Then, the VF PWU 2B provides a power of a frequency F corresponding to the air demand Q.
FIG. 2 is a time chart of the normal operation of the conventional VF PWU system shown in FIG. 1. When the control command 6 requires the increase of the air supply rate Q at time t.sub.1 and the decrease of the air supply rate Q to the original level at t.sub.3, the control unit 5A issues the corresponding output signals through the signal line 7A to the VF PWU 2B accordingly. Ordinarily, the VF PWU 2B is so designed that the output thereof changes at a fixed increasing rate or a fixed decreasing rate. Accordingly, the output frequency F of the VF PWU 2B changes gradually from the times t.sub.1 and t.sub.3 when the air supply rate increasing command and the air supply rate decreasing command are given respectively and reaches the frequencies corresponding to the commands with a delay at times t.sub.2 and t.sub.4 respectively. That is, when the air supply rate Q of the fan 4 needs to be changed, the revolving rate n of the motor 3 is controlled by changing the output frequency of the VF PWU 2B.
FIG. 3 is a time chart of the abnormal operation of the conventional VF PWU system shown in FIG. 1 where a trouble, such as the disconnection of the signal line 7A, the loosening of the terminal or the falling off of the terminal, has arisen. In such a case, as a matter of course, the control command 6 remains unchanged and commands a fixed air supply rate Q. If the signal line 7A is disconnected at time t.sub.5, no control signal is given to the VF PWU 2B. Consequently, the output frequency F of the VF PWU 2B changes, as explained with reference to FIG. 2, to a lower limit value with a delay causing the resultant decrease of the revolving rate n of the motor 3, hence the reduction of the air supply rate Q of the fan 4.
As the conventional VF PWU controlling system functions in the manner as described hereinbefore, a failure such as the disconnection of the signal line for transmitting a control signal given by the VF PWU control unit to the VF PWU causes the useless change in the output frequency of the VF PWU 2B, which disturbs the operation of the motor 3, hence the operation of the rotary machine, thereby to cause the resultant malfunction of the entire system.